1. Field of the Invention
The present invention relates to a phosphor, and more particularly, to a green oxide phosphor that has improved chemical stability as well as improved luminance.
2. Description of the Related Art
Generally, phosphor indicates a material emitting visible ray when the phosphor excited by an electromagnetic wave such as an ultraviolet ray, an electron ray or an X-ray, transits to a ground state back. Various phosphors have been developed. These phosphors have metal oxide, sulfide, acid sulfide, halide, or the like as a host lattice and emit near ultraviolet ray or visible ray due to the above-mentioned electromagnetic wave.
The phosphors are widely used in a fluorescent lamp, a radioactive ray intensifying screen, an indoor/outdoor decoration fluorescent tile, and a display device, such as a cathode ray tube (CRT), a vacuum fluorescent display (VFD) or a plasma display panel (PDP).
Particularly, in the PDP, a vacuum ultraviolet ray of 147 nm allows the phosphor to emit light so that an image is displayed. The vacuum ultraviolet ray is generated when inert mixed gases, such as He+Xe, Ne+Xe and He+Xe+Ne are discharged. Since the PDP can be easily made in a slim and large-sized structure, it attracts attention as a large-sized flat panel display.
Recently, the PDP begins to be produced commercially in Korea and Japan, and extends its market share, and the image quality of the PDP continues to be improved thanks to the advancing of its technology.
Referring to FIG. 1, a discharge cell of a three-electrode AC surface discharge type PDP includes a pair of sustain electrodes 9 formed on an upper substrate 1 and an address electrode X formed on a lower substrate 2.
Each of the pair of sustain electrodes 9 includes a transparent electrode 9a of Indium-Tin-Oxide (ITO) and a metal bus electrode 9b formed on an edge of the transparent electrode 9a and having a narrower width than the transparent electrode 9a. The metal bus electrode 9b is formed by sequentially depositing Cr/Cu/Cr and etching the deposited Cr/Cu/Cr. An upper dielectric layer 6 and a passivation layer 7 are deposited on the upper substrate 1 on which the pair of sustain electrodes 9 are formed, by using a screen printing process or a vacuum deposition process. Wall charges generated during plasma discharge are accumulated on the upper dielectric layer 6. The passivation layer 7 is formed on the upper dielectric layer 6 at a thickness of about 5000 Å so as to protect the upper dielectric layer 6 and the pair of sustain electrodes 9 from damages caused by sputtering during plasma discharge and to enhance a discharge efficiency of secondary electrons. In general, magnesium oxide (MgO) is used for the passivation layer 7.
In drawings, Y represents a scan sustain electrode and Z represents a common sustain electrode.
A lower dielectric layer 4 and a barrier rib 3 are formed on a lower substrate on which an address electrode X is formed. A phosphor 5 is formed on the surfaces of the lower dielectric layer 4 and the barrier rib 3 by a screen printing process. The address electrode X is perpendicular to the sustain electrode pair 9.
The barrier rib 3 is formed by a screen printing or a molding process to prevent ultraviolet rays and visible rays generated during discharge from leaking to an adjacent discharge cell. The phosphor 5 is excited by a vacuum ultraviolet ray to emit one visible ray of any one of red, green and blue. The vacuum ultraviolet ray is generated during plasma discharge of the mixed gases injected into the discharge cell.
In order to realize the gray scale of an image, the PDP is time-division driven by dividing one frame into several sub-fields having different emission frequencies. Each sub-field is divided into an initialization period, an address period and an sustain period. In the initialization period, a full screen is initialized. In the address period, a scan line is selected and a cell is selected in the selected scan line. In the sustain period, the gray scale is realized according to discharge rate.
For example, when an image is displayed in 256 gray scales, a frame period (16.67 ms) corresponding to 1/60 second is divided into eight sub-fields SF1 to SF8 as shown in FIG. 2. Each of the eight sub-fields SF1 to SF8 is divided into the initialization period, the address period and the sustain period as described above. The initialization period and the address period are the same in each sub-field while the sustain period increases at a rate of 2n (n=0, 1, 2, 3, 4, 5, 6, 7).
In the PDP operating as described above, the phosphor 5 is excited by the vacuum ultraviolet ray to emit light, and is classified into a red phosphor, a green phosphor and a blue phosphor according to the wavelength of the emitted light.
Referring to FIG. 3, the red phosphor widely used in the PDP has a composition of (Ygd) BO3:Eu3+, the blue phosphor has a composition of BaMgAl10O17:Eu2+, and the green phosphor has a composition of Zn2SiO4:Mn2+. Such phosphors are coated on the barrier rib 3 of the PDP and directly exposed to mixed gases filled in a discharge cell 10. Negative and positive charges 11 and 12 are present in the discharge cell 10. If the mixed gases emit vacuum ultraviolet ray during plasma discharge, each of the phosphors emits red, green and blue lights.
Then, since the green phosphor with the composition of Zn2SiO4:Mn2+ is long in afterglow time during which green light continues to be maintained after the green light is emitted, an afterimage is left on the PDP screen. Consequently, the conventional green phosphor deteriorates the display quality of the PDP. In addition, since the dielectric characteristic and the surface characteristic of the conventional green phosphor are not excellent, the conventional green phosphor has a discharge delay problem that discharge is not generated in time. Since the conventional green phosphor has a high threshold voltage to emit the light, that is, a high discharge voltage, its power consumption is increased. Hence, it is an urgent subject to develop the phosphor with a short afterglow time, excellent dielectric characteristic and excellent surface characteristic so as to improve the display quality of the PDP and to reduce the consumption power.
Furthermore, since the application of the phosphor as well as the application of the PDP is diversified and highly developed today, it is strongly required to develop a green phosphor that can solve the above-mentioned problems, and that has a higher luminance characteristic than the conventional phosphor, and is chemically stable under various application environments.